The kiwowatt hour (SI symbow kWh, kW⋅h or kW h) is a derived unit of energy eqwaw to 3.6 megajouwes. If de energy is being transmitted or used at a constant rate (power) over a period of time, de totaw energy in kiwowatt hours is de power in kiwowatts muwtipwied by de time in hours. The kiwowatt hour is commonwy used as a biwwing unit for energy dewivered to consumers by ewectric utiwities.

One watt is eqwaw to 1 J/s. One kiwowatt hour is 3.6 megajouwes,[1][2] which is de amount of energy converted if work is done at an average rate of one dousand watts for one hour.

The base unit of energy widin de Internationaw System of Units (SI) is de jouwe. The hour is a unit of time "outside de SI", making de kiwowatt hour a non-SI unit of energy. The kiwowatt hour is not wisted among de non-SI units accepted by de BIPM for use wif de SI, awdough de hour, from which de kiwowatt hour is derived, is.[3]

Ewectricaw energy is often sowd in kiwowatt hours. The cost of running an ewectric device is cawcuwated by muwtipwying de device's power in kiwowatts, by de running time in hours, by de price per kiwowatt hour. The unit price of ewectricity may depend upon de rate of consumption and de time of day. Industriaw users may awso have extra charges according to deir peak usage and de power factor.

The symbow "kWh" is commonwy used in commerciaw, educationaw, scientific and media pubwications,[4][5] and is de usuaw practice in ewectricaw power engineering.[6]

Oder abbreviations and symbows may be encountered:

"kW h" is wess commonwy used. It is consistent wif SI standards[7]. The internationaw standard for SI[3] states dat in forming a compound unit symbow, "Muwtipwication must be indicated by a space or a hawf-high (centered) dot (⋅), since oderwise some prefixes couwd be misinterpreted as a unit symbow" (i.e., kW h or kW⋅h). This is supported by a vowuntary standard[8] issued jointwy by an internationaw (IEEE) and nationaw (ASTM) organization, uh-hah-hah-hah. However, at weast one major usage guide[9] and de IEEE/ASTM standard awwow "kWh" (but do not mention oder muwtipwes of de watt hour). One guide pubwished by NIST specificawwy recommends avoiding "kWh" "to avoid possibwe confusion".[10]

"kW⋅h" is, wike "kW h", preferred by SI standards, but it is very rarewy used in practice.

The US officiaw fuew-economy window sticker for ewectric vehicwes uses de abbreviation "kW-hrs".[11]

Aww de SI prefixes are commonwy appwied to de watt hour: a kiwowatt hour is 1,000 W⋅h (symbows kW⋅h, kWh or kW h; a megawatt hour is 1 miwwion W⋅h, (symbows MW⋅h, MWh or MW h); a miwwiwatt hour is 1/1000 W⋅h (symbows mW⋅h, mWh or mW h) and so on, uh-hah-hah-hah. The kiwowatt hour is commonwy used by ewectricaw distribution providers for purposes of biwwing, since de mondwy energy consumption of a typicaw residentiaw customer ranges from a few hundred to a few dousand kiwowatt hours. Megawatt hours (MWh), gigawatt hours (GWh), and terawatt hours (TWh) are often used for metering warger amounts of ewectricaw energy to industriaw customers and in power generation, uh-hah-hah-hah. The terawatt hour and petawatt hour (PWh) units are warge enough to convenientwy express de annuaw ewectricity generation for whowe countries and de worwd energy consumption.

The terms power and energy are freqwentwy confused. Power is de rate of dewivery of energy. Power is work performed per unit of time. Energy is de work performed (over a period of time).

Power is measured using de unit watts, or jouwes per second. Energy is measured using de unit watt hours, or jouwes.

A common househowd battery contains energy. When de battery dewivers its energy, it does so at a certain power wevew, dat is, de rate of dewivery of de energy. The higher de power wevew, de qwicker de battery's stored energy is dewivered. If de power is higher, de battery's stored energy wiww be depweted in a shorter time period.

For a given period of time, a higher wevew of power causes more energy to be used. For a given power wevew, a wonger run period causes more energy to be used. For a given amount of energy, a higher wevew of power causes dat energy to be used in wess time.

An ewectricaw woad (e.g. a wamp, TV-set, or ewectric motor) has a rated power, which is usuawwy measured in watts. This is its running power wevew, which eqwates to de instantaneous rate at which energy must be generated and consumed to run de device. The amount of energy dat is consumed (at dat rate) depends on how wong de device is operated. However, its power reqwirement is constant whiwe running. The unit of energy used for residentiaw ewectricaw biwwing, de kiwowatt hour, shows de cumuwative amount of ewectricaw energy used during de biwwing period, regardwess of de power drawn at any moment during de biwwing period.

For anoder exampwe, when a wight buwb wif a power rating of 100 watts is turned on for one hour, de energy used is 100 watt hours (W⋅h), which is eqwaw to 0.1 kiwowatt hours. This same amount of energy wouwd wight a 50-watt buwb for 2 hours, a 40-watt buwb for 2.5 hours, or a 10-watt buwb for 10 hours. A power station's ewectricity output at any particuwar moment wouwd be measured in muwtipwes of watts, but its annuaw energy sawes wouwd be in muwtipwes of watt hours. A kiwowatt hour is de amount of energy eqwivawent to a steady power of 1 kiwowatt running for 1 hour, or 3.6 megajouwes.

Whereas individuaw homes onwy pay for de kiwowatt hours consumed, commerciaw buiwdings and institutions awso pay for peak power consumption, de greatest power recorded in a fairwy short time, such as 15 minutes. This compensates de power company for maintaining de infrastructure needed to provide peak power. These charges are biwwed as demand charges.[12]

Major energy production or consumption is often expressed as terawatt hours (TW⋅h) for a given period dat is often a cawendar year or financiaw year. A 365-day year eqwaws to 8,760 hours, derefore one gigawatt eqwaws to 8.76 terawatt hours per year. Conversewy, one terawatt hour is eqwaw to a sustained power of approximatewy 114 megawatts for a period of one year.

Power units measure de rate of energy per unit time. Many compound units for rates expwicitwy mention units of time, for exampwe, miwes per hour, kiwometers per hour, dowwars per hour. Kiwowatt hours are a product of power and time, not a rate of change of power wif time. Watts per hour (W/h) is a unit of a change of power per hour. It might be used to characterize de ramp-up behavior of power pwants. For exampwe, a power pwant dat reaches a power output of 1 MW from 0 MW in 15 minutes has a ramp-up rate of 4 MW/h. Hydroewectric power pwants have a very high ramp-up rate, which makes dem particuwarwy usefuw in peak woad and emergency situations.

The proper use of terms such as watts per hour is uncommon, whereas misuse[13] may be widespread.

Severaw oder units are commonwy used to indicate power or energy capacity or use in specific appwication areas.

Average annuaw power production or consumption can be expressed in kiwowatt hours per year; for exampwe, when comparing de energy efficiency of househowd appwiances whose power consumption varies wif time or de season of de year, or de energy produced by a distributed power source. One kiwowatt hour per year eqwaws about 114.08 miwwiwatts appwied constantwy during one year.

The energy content of a battery is usuawwy expressed indirectwy by its capacity in ampere-hours; to convert ampere-hour (A⋅h) to watt hours (W⋅h), de ampere-hour vawue must be muwtipwied by de vowtage of de power source. This vawue is approximate, since de battery vowtage is not constant during its discharge, and because higher discharge rates reduce de totaw amount of energy dat de battery can provide. In de case of devices dat output a different vowtage dan de battery, it is de battery vowtage (typicawwy 3.7 V for Li-ion) dat must be used to cawcuwate rader dan de device output (for exampwe, usuawwy 5.0 V for USB portabwe chargers). This resuwts in a 500 mA USB device running for about 3.7 hours on a 2500 mAh battery, not five hours.

The Board of Trade unit (BOTU) is an obsowete UK synonym for kiwowatt hour. The term derives from de name of de Board of Trade which reguwated de ewectricity industry untiw 1942 when de Ministry of Power took over.[14]

The British dermaw unit or BTU (not to be confused wif BOTU), is a unit of dermaw energy wif severaw definitions, aww about 1055 Jouwe or 0.293 watt hour. The qwad, short for qwadriwwion BTU, or 1015 BTU, is sometimes used in nationaw-scawe energy discussions in de United States. One qwad is approximatewy 293 TWh or 1.055 exajouwe (EJ).

In India, de kiwowatt hour is often simpwy cawwed a Unit of energy. A miwwion units, designated MU, is a gigawatt hour and a BU (biwwion units) is a terawatt hour.[15][16]

Burnup of nucwear fuew is normawwy qwoted in megawatt days per tonne (MW⋅d/MTU), where tonne refers to a metric ton of uranium metaw or its eqwivawent, and megawatt refers to de entire dermaw output, not de fraction which is converted to ewectricity.[citation needed]

^"Hawf-high dots or spaces are used to express a derived unit formed from two or more oder units by muwtipwication, uh-hah-hah-hah." Barry N. Taywor. (2001 ed.) The Internationaw System of Units.Archived June 3, 2016, at de Wayback Machine. (Speciaw pubwication 330). Gaidersburg, MD: Nationaw Institute of Standards and Technowogy. 20.

^"Guide for de Use of de Internationaw System of Units (SI)"(PDF). physics.nist.gov. Nationaw Institute of Standards and Technowogy. 2008. Archived(PDF) from de originaw on 3 June 2016. Retrieved 25 January 2017. Reference [4: ISO 31-0] suggests dat if a space is used to indicate units formed by muwtipwication, de space may be omitted if it does not cause confusion, uh-hah-hah-hah. This possibiwity is refwected in de common practice of using de symbow kWh rader dan kW ⋅ h or kW h for de kiwowatt hour. Neverdewess, dis Guide takes de position dat a hawf-high dot or a space shouwd awways be used to avoid possibwe confusion;